Field of the Invention
The present invention generally relates to sensing strain in materials and, more particularly, is concerned with a method of sensing strain in a material by driving an embedded magnetoelastic film-coated wire to saturation.
Description of the Prior Art
A variety of methods may be used to measure strain in materials. A common method uses the change in resistance with strain. Conventional foil sensors and silicon-based resistive sensors use this method, but both of these sensors are bulky. More recent types of sensors include fiber optics sensors which can be used to measure strain.
It is well-known that magnetoelastic materials are stress sensitive. Magnetoelastic materials will change their magnetic properties under an applied stress and thus have been employed to measure strain. Magnetoelastic-based sensors are disclosed in U.S. Pat. No. 3,774,134 to Kardashian et al. and in U.S. Pat. No. 3,863,501 to Janssen et al. In the Kardashian et al. and Janssen et al. patents, wires plated with magnetoelastic material are disclosed. The plated wires of the Kardashian et al. and Janssen et al. patents have a composition which is controlled around the circumference of the wire such that a portion of the wire will have a negative magnetoelastic effect and a portion of the wire will have a positive magnetoelastic effect. The plated wires of the Kardashian et al. and Janssen et al. patents are fabricated with non-zero magnetoelasticity. The plated wires of the Kardashian et al. and Janssen et al. patents can be used to monitor stress by virtue of one half of each wire having a negative magnet elasticity and the other half of each wire having a positive magnetoelasticity.
While prior art magnetoelastic-based sensors, such as those which are disclosed in the Kardashian et al. and Janssen et al. patents, may function satisfactorily in some uses, a need still remains for a more effective strain sensing means which will provide a more optimum solution for measuring strain in materials.
The present invention provides a method of sensing strain in a material which is designed to satisfy the aforementioned need. The strain sensing method of the present invention drives a magnetoelastic film on a conductive wire to saturation. At least one wire formed of a conductive core surrounded and coated externally by a magnetoelastic film is employed. The magnetoelastic film of the wire is fully magnetized at its saturation point which provides an optimum degree of sensitivity in measuring the strain of a material which surrounds the wire or, in other words, in which the wire is embedded.
Accordingly, the present invention is directed to a method of sensing strain in a material which comprises the steps of: (a) embedding in a material at least one wire formed of a conductive core and a magnetoelastic film coated around the conductive core and having a known level of saturation; (b) interconnecting an input lead to one end portion of the wire and an output lead to another end portion of the wire; (c) inputting an electrical current in the wire through the input lead so as to drive the magnetoelastic film of the wire to saturation such that the electrical impedance is sensitive to changes in strain in the material; and (d) outputting an electrical signal from the output lead being proportional to a strain imposed on the material surrounding the wire.
More particularly, the material is a composite material. The wire is a plurality of wire segments interconnected in series and embedded in the material. A plurality of input leads and a plurality of output leads are provided such that one input lead is interconnected to one end portion and one output lead is interconnected to another end portion of each of the wire segments. The electrical current is inputted in each of the wire segments through the input leads so as to drive the magnetoelastic film of each wire segment to saturation such that the electrical impedance is sensitive to changes in strain in the material and an electrical signal is outputted from each output lead proportional to a strain imposed on the material surrounding the wire segment to which the output lead is interconnected. The magnetoelastic film has a thickness ranging from about 1 micrometer to about 100 micrometers and is composed of an electrodeposited nickel and iron alloy. The magnetoelastic film is continuous and uniform around the conductive core of the wire and increases the impedance of the wire.
The present invention is also directed to a method of monitoring the curing of a material which comprises the steps of: (a) embedding in a material which is uncured at least one wire formed of a conductive core and a magnetoelastic film coated around the conductive core and having a known level of saturation; (b) interconnecting an input lead to one end portion of the wire and an output lead to another end portion of the wire; (c) inputting an electrical current in the wire through the input lead so as to drive the magnetoelastic film of the wire to saturation such that the electrical impedance is sensitive to changes in the extent of curing of the material; and (d) outputting an electrical signal from the output lead being proportional to the extent of curing of the material surrounding the wire.
The present invention is also directed to a method of limiting current from a source of electrical power which comprises the steps of: (a) providing a source of alternating current; (b) providing a load for receiving the alternating current; and (c) connecting between the source of alternating current and the load at least one wire formed of a conductive core and a magnetoelastic film coated around the conductive core for limiting a maximum amount of alternating current flowing to the load.